In my prior U.S. Pat. No. 3,861,855, there is disclosed a furnace having an automatic control of the efficiency of combustion. This furnace has a combustion chamber from which smoke, produced incidentally to combustion, is evacuated via a flue. A smoke density measuring apparatus is operatively associated with the flue. This apparatus includes a lamp directing light through the smoke and a photoelectric cell which responds to the strength of the light which has passed through the smoke.
The response of the photoelectric cell is employed in an electrical circuit to operate a motor or other such electromechanical device which, in turn, controls different combustion controlling elements such as, for example, fuel valves, air valves, dampers and so forth. The response of the photoelectric cell is more specifically utilized by placing the cell in series with a variable resistor or potentiometer to form in effect a voltage divider such that the junction between the photoelectric cell and potentiometer presents a voltage signal representative of smoke density. This signal is processed and used in the control of a motor which operates the aforesaid elements.
The use of the afore-noted potentiometer is to permit adjustment of the voltage divider arrangement to select a no-smoke or preferred smoke level. This assumes, however, constant conditions relative to the lamp and photoelectric cells which constant conditions do not in fact generally exist.